The invention relates to a bad frame indication of speech blocks in a wireless digital communication system, and more particularly, to a method and apparatus of determining the bad frame indication of speech blocks for identifying bad blocks received in a voice transmission in a wireless digital communication system.
The Global System for Mobile Communications (GSM), one of the most popular prevailing digital communication systems, provides speech service over a wireless environment. But the speech information, which is generated by digitizing and compressing the voice waveform, can be corrupted by severe fading conditions, which in turn causes the received speech blocks to become corrupted. Also, discontinuous transmission capability is activated if the speaking voice is inactive during the call. At the receiving side, each received block may contain corrupted or useless information for voice synthesis. This kind of corrupted or useless block needs to be marked as bad to notify the speech processing so as to prevent erroneous decoding, which would lead to annoying audio effects being generated for the listener.
Thus, the bad frame indication (BFI) decision is to recognize both the bad speech and the non-speech blocks as ‘bad’ ones effectively. Then these bad frames can be erased through error concealment. The traditional method for determining the BFI is based on the CRC result only. That means if the checksum of CRC is not equal to zero, the speech frame is considered to be bad. Unfortunately, in practice, the length of CRC parity bits is not long enough in GSM applications. And, in the fading environment, the speech block could be corrupted. In the speech decoder, the corrupted speech information leads to the unpleasant noise and degrades the speech quality severely. For example, if the Discontinuous Transmission (DTX) is regarded as a useful speech block, the clipping effect causes the listener to experience unpleasant noise. Therefore, some other strategies for combating bad and lost speech frames, such as error concealment, muting, and substitution, are needed.